The present invention relates to the field of semiconductor fabrication; more specifically, it relates to method for fabricating a buried collar in a trench.
One use for buried collars is for device isolation and suppression of vertical parasitic leakage in trench dynamic random access memory (DRAM) structures. The thickness of the collar is fixed by the operating voltage of the device. As dimensions shrink, the diameter of the trench is shrinking faster than the operating voltage so a higher and higher percentage of the trench cross-section is taken up by the collar. Present techniques for fabricating collars do not work if the trench diameter is about three times the collar thickness.